Combustion process and apparatus therefore containing separate injection of fuel and oxidant streams

ABSTRACT

A burner assembly having improved flame length and shape control is presented, which includes in exemplary embodiments at least one fuel fluid inlet and at least one oxidant fluid inlet, means for transporting the fuel fluid from the fuel inlet to a plurality of fuel outlets, the fuel fluid leaving the fuel outlets in fuel streams that are injected into a combustion chamber, means for transporting the oxidant fluid from the oxidant inlets to at least one oxidant outlet, the oxidant fluid leaving the oxidant outlets in oxidant fluid streams that are injected into the combustion chamber, with the fuel and oxidant outlets being physically separated, and geometrically arranged in order to impart to the fuel fluid streams and the oxidant fluid streams angles and velocities that allow combustion of the fuel fluid with the oxidant in a stable, wide, and luminous flame. Alternatively, injectors may be used alone or with the refractory block to inject oxidant and fuel gases. The burner assembly affords improved control over flame size and shape and may be adjusted for use with a particular furnace as required.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a combustion process and an apparatustherefor that provides means of introducing a fuel and an oxidant inseparate streams in the combustion chamber of a furnace, so that thefuel burns with the oxidant in a wide, luminous flame, and whereby thecombustion of the fuel with the oxidant generates reduced quantities ofnitrogen oxides (NO_(x)).

2. Related Art

Industrial high temperature processes, such as glass or frit melting,ferrous and non ferrous materials smelting, use large amounts of energyto transform a variety of raw materials into a hot molten product, thatis then cast, formed or otherwise disposed of in further stages of theindustrial process. This operation is generally performed in largefurnaces, that can produce as much as 500 tons per day of moltenmaterial. Combustion in the furnace of a fossil fuel, such as naturalgas, atomized fuel oil, propane, or the like, with an oxidant thatcontains oxygen is a preferred method of supplying the energy. In somecases, the combustion is supplemented by electric heating. Most of thetime, the fuel and the oxidant are introduced in the furnace throughburners, in order to generate flames. The transfer of energy from theflames to the material to be melted results from the combination ofconvection at the surface of the material, and radiation to the surfaceor into the material if it is transparent to the radiation. Flames thatare highly radiant (usually referred to as luminous flames), are usuallypreferred, because they provide better heat transfer and, thus, higherfuel efficiency.

For flame heating, it is also very important to have the energy from theflame evenly distributed above the surface of the material to be melted.Otherwise, hot and cold regions may co-exist in the furnace, which isnot desirable. The quality of products manufactured with material meltedin such a furnace is often poor. For example, in a bath of molten glass,there may be glass stones in cold regions, and acceleratedvolatilization of glass in hot regions. Also, broad flames are preferredbecause they yield a better bath coverage.

In many countries, particularly the United States, increasinglystringent regulations are being promulgated regarding emissions ofNO_(x). It is, therefore, important to develop combustion techniqueswherein NO_(x) formation is limited. In very high temperature processes,NO_(x) formation is promoted by long residence times of oxygen andnitrogen molecules in hot regions of the flame and the furnace. The useof substantially pure oxygen (about 90% O₂ or higher) instead of air asthe oxidant has proven to be very successful in reducing the NO_(x)emissions by as much as 90%, since all nitrogen is eliminated. However,substitution of air by substantially pure oxygen increases the flametemperature, and thus creates regions in the furnace where thereactivity of nitrogen with oxygen is high, and wherein the formation ofNO_(x) may proportionally increase, even though it is globally decreasedwhen compared to combustion with air. Also, it is impossible in practiceto eliminate all nitrogen from a furnace, because industrial furnacesare not tight to air leaks, the fuel usually contains some nitrogen, andoxygen from non-cryogenic sources, such as oxygen produced by a VacuumSwing Adsorption plant (VSA) contains a small residual nitrogenconcentration.

Conventional methods of combusting fuel and oxygen for heating furnacesutilize post mix oxy-fuel burners. Conventional oxy-fuel burners have ametallic body with inlets for a fuel and an oxidant with a highconcentration of molecular oxygen, and means to transport the streamswith separate coaxially oriented channels to multiple injectors locatedat the burner tip. These burners generate high temperature flames withthe shape of a narrow pencil at the burner tip, which needs to belocated far enough into the furnace, to avoid or reduce overheating ofthe furnace walls. As a consequence of the high temperatures encounteredin melting furnaces, one important drawback of these burners is the needfor cooling, usually a jacket where a circulating fluid such as waterprovides the cooling. Such a burner is described, for example, inBritish Patent 1,215,925. Severe corrosion problems for the coolingjacket can arise particularly when the furnace atmosphere containscondensable vapors.

The gas cooled oxy-fuel burner is an improvement of the water-cooledburner. The body of the burner is protected from the furnace radiationby a refractory brick often referred to as a burner block, thatpossesses a substantially cylindrical cavity that opens onto thefurnace. The burner is usually mounted at the back of the cavity, and itusually contains concentric injectors of fuel and oxidant located in thecavity, recessed from the furnace inner wall. The brick and the burnerare cooled by a peripheral annular flow of gas, usually the oxidant gas.Such burners are described e.g. in U.S. Pat. No 5,346,390 and U.S. Pat.No. 5,267,850. With this type of burner, combustion starts in the burnerblock before reaching the furnace. Thus, the flame is confined in anddirected by the cylindrical cavity as a narrow axisymmetric jet, andprovides insufficient covering of the melt in the furnace. These flameshave high peak temperatures and generate relatively large amounts ofNO_(x), because there is a direct contact between the oxygen and thefuel without dilution by the combustion products.

Another drawback of these gas cooled burners is that the flame mayoverheat and damage the furnace refractory wall because it starts in thewall itself. Also recirculation zones under the flame itself tend toaccelerate refractory wear when the furnace atmosphere chemically reactswith the refractory material of the furnace wall which may reduce thefurnace lifetime.

British Patent 1,074,826 and U.S. Pat. No 5,299,929 disclose burnerscontaining alternated multiple oxygen and fuel injectors in parallelrows in order to obtain a flatter flame. Although this brings animprovement in terms of coverage of the melt, these burners stillproduce relatively large amounts of NO_(x). Another drawback of theseburners is that they are mechanically complex to build in order toobtain a flat flame.

It is also known to inject fuel and oxidant by separate injectors into acombustion chamber to generate flames detached from the furnace wall,with the aim of reducing refractory wear. One such apparatus isdescribed in U.S. Pat. No. 5,302,112 wherein fuel and oxidant jets areinjected at a converging angle into a furnace, which yields good mixingof the oxidant and fuel gases at the converging point of the two jets,thus enhancing the combustion rate but shortening the flame. However,the flame of such a burner has a high peak temperature and largequantities of nitrogen oxides are created in the furnace. To decreasethis high peak temperature and significantly reduce formation of NO_(x)it has been suggested in U.S. Pat. No. 4,378,205 to inject the fueland/or the oxidant jets at very high velocities and to use separateinjections of fuel and oxidant gases wherein the fuel and/or the oxidantjets entrain combustion products contained in the furnace atmosphere,and are diluted before the actual combustion between the fuel and theoxidant. However, the flames generated by these burners are almostinvisible, as disclosed therein, col. 9, lines 58-65. It is, thus,extremely difficult for a furnace operator to determine and/or controlthe location of the combustion zones, and whether or not the burnerapparatus is actually turned on, which may be hazardous. For certainapplications such as glass melting, it is also generally recognized thatluminous flames are desirable, because heat transfer from such flames ismore efficient than for invisible flames. Another drawback of thisburner is that the entrainment of combustion products promotes strongrecirculation streams of gases in the furnace, which in turn acceleratesthe wear of the refractory walls of the furnace.

Another technique used to improve the heat transfer from a flame to aload is disclosed in U.S. Pat. Nos. 4,909,733 and 4,927,357, where arate enhancing gas, generally oxygen, is injected through a nonaxisymmetric lance between a flame and the furnace load. With thistechnique, the flame temperature is increased, which results in highernitrogen oxide formation. Also, according to the above cited inventions,the rate enhancing gas needs to be injected at high velocity in order todisplace the flame towards the load. As mentioned before, this promotesstrong recirculation streams of gases in the furnace, which in turnaccelerates the wear of the refractory walls of the furnace.

Also, the use of high velocity oxidant jets requires the use of a highpressure oxidant supply, which means that the oxidant gas needs to beeither produced or delivered at high pressure (the fuel gas is usuallyat relatively high pressure) or that the oxidant gas, such as the lowpressure oxygen gas usually supplied by a VSA unit, has to berecompressed before being injected into the furnace.

Melting furnaces such as glass furnaces represent a high capitalinvestment. Thus it is desirable to extend the lifetime of a furnace asmuch as possible while maintaining productivity. One of the agingfactors of a furnace is superstructure temperature: for example, it hasbeen demonstrated that the rate of wear and corrosion of a glass furnacecrown was accelerated when the furnace was operated at high temperature.This can oblige the glass maker to repair the furnace prematurely, or toreduce the furnace pull rate at the end of the furnace campaign in orderto prevent a catastrophic failure. In the case of a furnace equippedwith oxy-fuel burners that produce generally high temperature flames, itis very important that the flames are not deflected towards the crownwhich would result in local hot spots. Such situations are known tooccur for unstable flames that are deflected by the complex flow patternof the combustion products in a furnace. For example, low momentumburners where the fuel and oxidant are injected at a low velocity in afurnace, overcome the drawbacks related earlier of high velocityburners, but tend to produce unstable flames. A combustion method thatwould prevent flame lofting and reduce furnace crown operatingtemperature would be particularly valuable for the industrials.

Thus, a need exists for a burner which may operate at low pressure,particularly for the oxidant gas, while producing a wide, flat, stable,luminous flame with reduced NO_(x) emissions, and which affords a mannerof controlling flame length so as to adapt the flame to the furnace inwhich it is used.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide methods andapparatus for combustion of a fuel with oxygen contained in an oxidantgas, wherein fuel is distributed in at least two streams injected in thecombustion chamber of a furnace and most of oxidant required forcomplete combustion of the fuel is injected through at least one,preferably one or two, elongated orifice (such as a generally ovalorifice depicted in the drawings) which axis along the orifice largestdimension (hereafter sometimes referred to as the major axis) isparallel to the surface of the material to be heated, in such mannerthat the stream of oxidant emerging from the elongated orifice convergestoward the fuel streams in order to generate a broad flame parallel tothe surface of the material to be heated. Two adjacent fuel streams makean angle ranging from 0° to about 15°, preferably ranging from 0° toabout 10°. The stream of oxidant flowing out of the at least oneelongated orifice, referred to as main oxidant, converges toward thefuel streams with an angle ranging from 0° to about 45°, preferably fromabout 2.5° to about 10°. The elongated orifice aspect ratio (maximumwidth [major axis] divided by maximum height [or minor axis]) preferablyranges from about 2 to about 8, more preferably from about 4 to about 6.

In preferred arrangements of the present invention, the fuel streams aresubstantially parallel to the surface to be heated, or oriented relativeto the surface to be heated with an angle not exceeding +10° or -10°,and the main oxidant stream converges towards the fuel streams and thesurface to be heated.

Another object of the present invention is to provide flames to heat upa melt contained in a furnace and to protect the crown of said meltingfurnace from overheating. Indeed, according to this aspect of theinvention, the effect of the stream of oxidant exiting the generallyrectangular orifice is to maintain the flame close to the melt andprevent the flame from lofting.

Another object of the invention is a method and apparatus for supplyingsecondary oxidant around the at least two fuel streams in order increaseflame luminosity by initiating combustion of the fuel before the mainoxidant stream intersects the fuel streams in the combustion chamber,and by creating a fuel rich mixture where significant amounts of sootare formed. Subsequent combustion of the fuel rich mixture with sootwith the main oxidant stream yields a luminous flame that providesefficient heat transfer. The flow of secondary oxidant is such that thesecondary oxidant supplies between 0 and 50% of the total amount ofoxidant required to obtain complete combustion of the fuel. Preferably,the amount of the secondary oxidant supplies between 0 and 25% of thetotal amount of oxidant required to obtain complete combustion of thefuel. The main oxidant and the secondary oxidant can be of differentnature: for example, the main oxidant can be industrially pure oxygen(oxygen concentration greater than 88%), and the secondary oxidant canbe ambient air.

According to an aspect of the present invention, a means is provided tovary the flame luminosity and flame shape by changing the flows of mainoxidant and secondary oxidant in such fashion that the total amount ofoxygen in the main oxidant flow and in the secondary oxidant flow issufficient to insure complete combustion of the fuel.

In preferred arrangements of the present invention, the main oxidant andthe secondary oxidant are provided by the same source, and the flameluminosity and shape are altered by changing the distribution of oxidantamong the main oxidant stream flowing through the generally rectangularorifice and the secondary oxidant flowing around the at least two fuelstreams. With this means, the flame luminosity increases as the amountof soot formed in the fuel rich mixture increases, and the flamegeometry is modified when the mixing conditions of the fuel and theoxidant are modified.

Additionally, it is an object of the present invention to provide acombustion method that generate flames with low peak temperatures, andthus reduce the emissions of nitrogen oxides by the combustion process.

It is also an object of the present invention to provide methods andapparatus for combustion of a fuel with an oxidant gas that contains atleast 50% of oxygen.

An important aspect of the present invention is provided by a burnerassembly comprising:

a) a refractory block having a cold end and a hot end, and furtherhaving at least one cavity for injection of fuel and one cavity forinjection of a main oxidant, the latter cavity ending on the hot end ofthe refractory block by an elongated opening having its major axisgenerally parallel to material to be heated,

b) a mounting bracket assembly removably attached to the cold end of therefractory block,

c) a metallic burner assembly attached to the refractory block by meansof said mounting bracket, the metallic burner assembly comprising atleast one oxidant inlet, and at least two oxidant outlets, the firstoxidant outlet opening on said cavity for injection of the main oxidant,the second oxidant outlet supplying secondary oxidant to the at leastone cavity for injection of fuel to initiate the combustion of the fuelclose to the hot face of the refractory block, the secondary oxidantfurther creating a protective layer of oxidant gas along the insidewalls of the at least one fuel cavity that prevents chemical reactionsbetween the refractory block material and the fuel that would eventuallydamage the burner block,

d) a fuel distributor assembly attached to the burner body comprisingone fuel inlet and fuel distribution means, fuel distribution meansextending into the least one cavity for injection of the fuel, andproviding the at least two fuel streams.

Another aspect of the present invention is the burner assembly describedin the above where the main oxidant and the secondary oxidant have thesame chemical composition, that further comprises splitting means todistribute the oxidant flow in the at least two oxidant outlets.

Other aspects of the invention pertain to the inside geometry of themain oxidant cavity geometry for a burner block of the previouslydescribed burner assembly.

Further aspects of the invention will become apparent after review ofthe following description and claims.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIGS. 1a and 1b represent schematic perspective views of burner blocksof the invention;

FIG. 2 represents a side sectional view of the burner block of FIG. 1aor 1b, through the section indicated "A--A" of FIG. 1b, illustrating theinner geometry of the main oxidant cavity (9) comprises four sections;

FIG. 3 represents an alternate embodiment of the inner geometry of themain oxidant cavity (9), wherein the diverging angle of section (12) isequal to (C), the diverging angle of section (11);

FIGS. 4, 5, and 6 represent schematic perspective views of burner blocksof the invention;

FIGS. 7a and 7b represent side sectional views of other refractoryblocks of the invention, illustrating preferred cavities (8) for thefuel injectors where the orifice (3) diameters are larger than thediameters of the remaining of the cavities;

FIGS. 8a, 8b and 8c represent front elevation views of burner blocks ofthe invention, wherein natural gas injectors are placed in cavities; and

FIGS. 9, 10, 11 and 12 illustrate in side sectional elevations of threeembodiments according to the present invention wherein there is provideda burner assembly in conduction with a refractory block.

DESCRIPTION OF PREFERRED EMBODIMENTS

The term "fuel", according to this invention, means, for example,methane, natural gas, liquefied natural gas, propane, atomized oil orthe like (either in gaseous or liquid form) at either room temperature(about 25° C.) or in preheated form. The term "oxidant", according tothe present invention, means a gas containing oxygen that can supportcombustion of the fuel. Such oxidants include air, oxygen-enriched aircontaining at least 50% vol. oxygen such as "industrially" pure oxygen(99.5%) produced by a cryogenic air separation plant, or non-pure oxygenproduced for example by a vacuum swing adsorption process (about 88%vol. oxygen or more) or "impure" oxygen produced from air or any othersource by filtration, adsorption, absorption, membrane separation, orthe like, at either room temperature or in preheated form. It is alsoimportant to note that, although in most instances it is preferred thatthe main and secondary oxidants be the same in chemical composition,they could be different. That is, the secondary oxidant could be airwhile the primary oxidant is industrially pure oxygen, or vice versa; orthe secondary oxidant could be impure oxygen while the primary oxidantis industrially pure oxygen, or vice versa.

The principle of operation of the combustion method of the inventionwill be more apparent after the following description of a number ofembodiments of the invention.

FIGS. 1a and 1b represent schematic perspective views of preferredburners (sometimes referred to herein as "burner blocks") (1) of theinvention. In the particular arrangement of FIG. 1a, fuel is injected inthe combustion chamber of a furnace (2) through two outlets (3) locatedin a burner block hot face (4). The axis of the fuel streams out ofburner block (1) are in the same plane, and make an angle (A) rangingfrom 0° (parallel arrangement) to about 30° with one another, (A)preferably ranging from 0° to about 10°. Most of the oxidant requiredfor the combustion of the fuel is injected through an elongated orifice(5) located in hot face (4) of burner block (1). In the embodiment shownin FIGS. 1a and 1b, elongated orifice (5) is a slot. The flow of oxidantcoming out of slot (5) makes an angle (B) with the direction of the fuelstreams ranging from 0° to about 20°. Preferred (B) angles are in therange from about 2.5° to about 10°. The slot aspect ratio (maximum widthdivided by maximum height) ranges from about 2 to about 8, preferablyfrom about 4 to about 6.

In FIG. 1b, the fuel is injected through three outlets (3) located inburner block hot face (4). The axis of the fuel streams out of burnerblock (1) are in the same plane, and make an angle (A) ranging from 0°to about 30° with one another. With the burner of FIG. 1b, it ispossible to spread the fuel in a sheet, and thus to generate a wide andflat combustion zone.

FIG. 2 represents a side sectional view of the burner block of FIG. 1aor 1b, through the section indicated "A--A" of FIG. 1b, illustrating theinner geometry of the main oxidant cavity (9) comprising four sections.The fuel streams originate from injectors (6) located in cylindricalcavities (7) of the burner block that open on the outlet orifices (3).Preferably the cavities are identical and located in a symmetricalarrangement relative to the slot (5). Secondary oxidant flows in thepassageway (8) situated between the injectors (6) and the cavities (7).The amount of secondary oxidant supplies from 0% up to about 50% of thetotal amount of oxygen required to completely combust the fuel. It wasfound that when the secondary oxidant provided more than 20% of theoxygen required for complete combustion of the fuel, the flame producedby the burner had a tendency to split in separate flames at the outletof the burner block, which is detrimental to the flame length. Thus,configurations where less than 20% of the oxygen required for completecombustion of the fuel are preferred. Preferably, injectors (6) arecentered in cavities (7), and recessed from hot face (4) of block (1) bya distance ranging from 0 to about 2 times the diameter of orifice (3)of the cavities.

The inner geometry of the main oxidant cavity (9) comprises preferablyfour sections. The first section (10) is generally cylindrical; thesecond section (10a) is generally cylindrical, of the same diameter asthe first section; the second section (10a) makes the angle (B) with theaxis of the first section; continuously attached to second section (10a)is a third section (11), generally conical with an angle (C) rangingfrom about 10° to about 120°, preferably ranging from about 10° to about45°; a fourth section (12) connects third section (11) with main oxidantorifice (5).

A preferred configuration for sections (10), (10a), (11), and (12) isshown in the sectional view of the block of FIG. 2 shown in FIG. 3: thediverging angle of section (12) is equal to (C), the diverging angle ofsection (11).

In alternate preferred arrangements of the invention, the means togenerate the at least two fuel streams are installed in the same cavityof a burner block. Such an arrangement is illustrated in FIG. 4, wheretwo fuel injectors (6) are placed in a single cavity (7) of the burnerblock. Secondary oxidant flows in the passageway comprised between thefuel injectors (6) and the cavity (7).

Another such arrangement is illustrated in FIG. 5 where a liquid fuelinjector (13) terminating with least two liquid fuel orifices (14) thatgenerate separate fuel streams is placed in cavity (7).

FIG. 6 represents an embodiment of the present invention similar to theembodiment of FIG. 1 but designed to use several fuels, where provisionfor an alternate fuel injector is made by placing an additional orifice(15) in the burner block: in one such embodiment, when fuel gas is used,the fuel is injected through orifices (3), and orifice (15) is not used;when a liquid fuel such as fuel oil is used, the fuel is injectedthrough orifice (15), with orifices (3) left unused.

When natural gas is used as a fuel, at nominal firing rate of theburner, the fuel velocity at the tip of the injectors (6) ranges fromabout 20 ms⁻¹ to about 150 ms⁻¹, preferably from about 30 ms⁻¹ about 80ms⁻¹. When the oxygen concentration of the oxidant is greater than 88%,the oxidant velocity at the orifice (5) ranges from about 5 ms⁻¹ toabout 80 ms⁻¹, preferably from about 10 ms⁻¹ to about 25 ms⁻¹.Preferably, the ratio of natural gas velocity to main oxidant velocityis ranges from about 2 to about 4. It was found that a burner of thepresent invention designed for a given nominal firing rate could be usedfrom 30% to 250% of its nominal rating.

FIGS. 7a and 7b represent alternate preferred cavities (8) for the fuelinjectors where the orifices (3) diameters are larger than the diametersof the remainder of the cavities (8). This provides improved protectionof the injector tips (6a) from the hot furnace environment by recessingthe injectors (6) farther from hot face (4) of the cavity withoutoverheating block (1). In FIG. 7b it is noted that the fuel cavitiespreferably have rounded or contoured edges at the exit point from theblock.

With the combustion method of the invention, fuel is injected in atleast two streams above the surface to be heated (furnace load). Thefuel is thus spread above the load in order to obtain a uniform heatflux distribution on the load. Increasing the angle between the fuelstreams in a similar fashion as increasing the angle (A) in FIGS. 1a and1b results in a wider combustion zone. However, as will be reportedlater, it was found that increasing the angle between fuel streamsbeyond 5° resulted in separate flames, which is not desirable because itdisturbs the uniformity of the combustion zone, a factor important whenthe load is molten glass. Also, increasing the angle between the fuelstreams yields a significant reduction of the flame length.

The main source of oxidant for the combustion of the fuel is theelongated orifice represented in FIGS. 1a and 1b, 4, 5, 6 by an ovalslot 5. The main stream of oxidant (in other words, the oxidantemanating from slot (5)) is oriented toward the streams of fuel with anangle (B), and is also oriented to the surface to be heated. Reducingthe angle (B) delays the mixing between the main oxidant and the fuel,which results in a longer combustion zone. However, very small (B)angles are not desirable, because the combustion zone becomes unstable.On the other hand, increasing the angle (B) increases the flamestability, but reduces the flame length, and pushes the flame towardsthe load. Preferably, it was found that (B) should range from about 2.5°to about 10° when one wants to avoid that the flame approached thefurnace load. Larger (B) angles can be found valuable in someapplications where direct contact of the flame with the surface to beheated is looked for, for example in the production of ferrous andnon-ferrous metals.

The effect of the main oxidant stream is to maintain the flame below theplane of the burner, to prevent the flame from lofting toward thefurnace crown (furnace crowns are present in, for example, glass tankfurnaces), and effectively to reduce the crown temperature, because theenergy is preferentially delivered to the load. Also, the combustionzone is preferentially pushed inside the furnace far from the sidewalls,which results in lower sidewall temperatures. With the combustion methodof the invention, the mixing of oxidant and fuel is staged, thusresulting in a lower flame temperature and low nitrogen oxides emissionrates.

Additional benefits provided by secondary oxidant injection are animproved cooling of the gas injectors by the gas flows, and the creationof a protective layer of oxidant gas along the inside walls of the fuelcavities that prevents chemical reactions between the refractory burnerblock material and the fuel gas. Such reactions are due to the partialthermal decomposition of the fuel containing carbon and hydrogen intocarbon atom C and hydrogen gas H₂, and the subsequent reactions betweenC and H₂ with the refractory materials. For refractories that containsilica, the intermediate reactions that yield loss of silica are:

    SiO.sub.2(s) +C.sub.(s) =SiO.sub.(g) +CO.sub.(g)

    SiO.sub.2(s) +C=SiO.sub.(g) +CO.sub.2

With hydrogen, the reaction is:

    SiO.sub.2(s) +H.sub.2 =SiO.sub.(g) +H.sub.2 O

In both cases, the suboxide of silica (SiO) is volatilized, andrecondenses in the combustion chamber, where additional oxygen is found.Other reactions are known to occur between silica and carbon, in thepresence of nitrogen, that produce silicon carbide (SiC.sub.(s)),silicon nitride (Si₃ N₄(s)), and silicon oxynitride (SiN₂ O.sub.(s)),all of which alter the refractory structure, and reduce the burner blocklifetime. With alumina, similar kinds of reactions occur at highertemperatures, with such products as Al₄ O₄ C.sub.(s), AlN.sub.(s), Al₄C₃(s), and the AlO.sub.(g) and Al₂ O.sub.(g) gases.

With the exception of fused zirconia, all refractory materials used formanufacturing burner blocks can be affected by the reduction mechanismsdescribed above, because all contain silica and alumina. Injecting thesecondary oxidant around the fuel streams along the burner blockcavities provides protection of the burner block from the fuel, bypreventing carbon and hydrogen to be in contact with the refractorymaterial.

The combustion method of the invention was tested at 1.7 MMBtu/hr (500kilowatt) firing rate in a 4 meters long, 1 square meter cross sectionhigh temperature pilot furnace. The flame geometry, the flame stabilityand the flame luminosity were monitored with a video camera mounted on aperiscope located in the roof of the furnace. A blue filter was insertedin front of the camera in order to eliminate part of the radiationemitted by the high temperature furnace walls. For the purpose of theevaluation of the combustion method, a prototype burner was built, witha main oxidant orifice (5) in the shape of a generally rectangular slotwith rounded edges of dimensions 4 inch (101.6mm) in width by 0.7 inch(17.8mm) in height. The oxidant used for both the main oxidant flow andthe secondary oxidant flow was 99.95% purity oxygen. The main oxidantvelocity at the outlet of the slot was close to 15 ms⁻¹. Natural gasinjectors (6) were placed in cavities (3), as indicated in FIG. 8a. Byusing two different sets of injectors, it was possible to change thenatural gas velocity at the outlet of the injectors from 29 ms⁻¹ to 55ms⁻¹. For the smallest injectors the diameter of the cavities (3) thatwere used for the tests were 0.824 inch (20.9mm) and 1.049 inch(26.6mm). Only the larger fuel cavities (1.049 inch [26.6mm]) could beused for the largest natural gas injectors. The distance (d) between thegas injectors was fixed at 4.5 inch (114.3mm). The distance (H) betweenthe main oxidant slot and the fuel injectors could be varied from 1.75inch (44.4mm) to 4.5 inch (114.3mm). The angle (A) could be varied from0 to 5 degrees, and the angle (B) could be varied from 0 to 10 degrees.

By injecting secondary oxidant around the fuel injectors, whilemaintaining constant the total amount of oxidant supplied to the burner,it was observed (unaided human eye) that the flame luminosity wasincreased. As little as 3% secondary oxidant provided a noticeable(unaided human eye) improvement in flame luminosity. It is estimatedthat the maximum flame luminosity was obtained with about 5% of thetotal oxidant flowing around the fuel injectors. This result isinterpreted by the partial combustion of the fuel in fuel richconditions that occurs between the fuel and the secondary oxidant thatpromotes soot formation. As the secondary oxidant flow was increasedabove 5% of the total oxidant, it was found that the flame luminositydecreased, and that the flame became shorter. For those tests, theamount of secondary oxidant ranged from about 3% to about 13% of thetotal oxidant. This resulted in a more intense mixing between fuel andthe increasingly higher velocity secondary oxidant flow, that tended toprevent soot formation, and to shorten the combustion zone.

The nitrogen oxide (NOx) emission rate did not increase by more than 10%when the amount of secondary oxidant was increased in the indicatedrange: at 3% secondary oxidant, the NOx concentration was 945 ppm, andthe maximum NOx concentration observed was 1035 ppm with increasedsecondary oxidant flow. In similar operating conditions, a tube in tubeoxy-fuel burner produced about 1800 ppm NOx. For these tests, no attemptwas made to achieve the lowest NOx emissions by suppressing all sourcesof nitrogen into the combustion chamber other than the nitrogennaturally present in the natural gas: in particular, the furnacepressure was slightly positive, but not high enough to prevent all airinfiltration, and some nitrogen was injected in the furnace to purge theperiscope lens.

It was also found that the height of the flame versus the load waschanged when changing the secondary oxidant flow: as the secondaryoxidant flow was increased, the flame moved farther from the load. Thisis a consequence of the higher momentum of the gas streams ejected fromthe fuel cavities in a direction substantially parallel to the furnaceload. It was also found that increasing the secondary oxidant flowresulted in higher temperatures near the burner block, which indicates afaster release of heat from the flame. Thus, by acting on thedistribution of the oxidant flow between the main oxidant stream and thesecondary oxidant streams, it is possible to change the flame length,the flame luminosity, the flame distance relative to the load, and theflame heat transfer distribution.

Increasing the angle (A) between adjacent fuel streams resulted in ashorter flame. However, when attempting to increase the angle betweenfuel streams beyond 5°, it was observed that the flame was replaced byseparate flamelets, which was not found acceptable because it disturbedthe uniformity of the combustion zone. Also, increasing the anglebetween the fuel streams yielded a reduction of the flame length.

Reducing the angle (B) between the direction of the main oxidant flowand the fuel natural gas flow appeared to delay the mixing between themain oxidant and the fuel, which resulted in a longer flame. Very small(B) angles were not found to be desirable, because the combustion zonebecame unstable. On the other hand, increasing the angle (B) increasedthe flame stability, but reduced the flame length, and pushed the flametowards the load. Preferably, it was found that (B) should range fromabout 2.5° to about 10° when one wants to avoid that the flameapproaches the furnace load.

When changing the distance (H) between the natural gas injectors and themain oxidant injectors, it was found that a distance of at least 3" wasnecessary to maintain the flame stability.

Increasing the natural gas velocity was also found to increase the flamestability. However, for a given natural gas velocity, there was noapparent influence of the fuel cavity diameter on the flame stability.Thus, the velocity of the secondary oxidant did not seem to have astrong effect on the flame stability.

The combustion burner of the invention depicted in FIG. 8b was alsotested at the 1.7 MMBtu/hr (500 kilowatt) scale in the high temperaturepilot furnace, with a prototype burner having a main oxidant orifice ofa generally oval shape with rounded edges, and of dimensions 4 inches(101.6mm) in width by 0.7 inch (17.8mm) in height. Natural gas wasinjected with three injectors centered 0.824 inch (20.9mm) in diametercavities. The corresponding natural gas velocity was 37 ms⁻¹. Thedistance d between adjacent gas injectors was 2 inches (50.8mm). Thedistance H between the natural gas injectors and the main oxidantinjector could be varied between 1.75 inch (44.5 mm) and 4 inches (101.6mm). The angle B between the direction of the main oxidant flow and thedirection of the gas flow could be varied from 5° to 10°. With thisconfiguration, it was possible to achieve wider flames than the flamesof the configuration of FIG. 8a, without creating separate flamelets.The influences of variations of the geometric parameters A, B, H, and ofthe distribution of the oxidant between the main flow and the secondaryflow on the flame geometry, the flame stability, and the flameluminosity that were observed with the configuration of FIG. 8a, wereconfirmed with the configuration with 3 fuel injectors.

Preferably, the natural gas injectors should be recessed from the burnerblock hot face in order to protect them from the heat of the furnace.The distance from the tip of the injector (6a, FIG. 7) to the burner hotface (4) should not exceed 2 times the largest internal diameter of thecavity, otherwise there is a risk of having the inner wall of the cavitybeing in contact with the combustion products of the fuel with thesecondary oxidant, especially if the fuel injector is not perfectlycentered in the cavity.

The previous burner configurations were compared to oxy-fuel burnerssimilar in design but where the main oxidant slot (5) was replaced bytwo holes or ovals (5a) and (5b) placed next to one another, spaced by 4inches (101.6mm) as in FIG. 8c. It was found that the burner with thesingle oval slot exhibited a more stable flame. In particular, the flamefrom the burner with the two oxidant holes or ovals lacked stability onthe sides of the flame (wings); this instability was completelyeliminated when replacing the two holes by the single oval slot.

An embodiment according to the present invention is provided by a burnerassembly such as in FIGS. 9a and 9b comprising:

a) a refractory burner block (1) having a cold end (16) and a hot end(4), and further having at least one cavity (7) for injection of thefuel in at least two streams, one cavity (9) for injection of most ofthe oxidant necessary for the complete combustion of the fuel, thelatter cavity ending on hot end (4) of block (1) by an elongated opening(5) such as a generally rectangular orifice,

b) a mounting bracket assembly (17) removably attached to the cold endof the refractory block,

c) a metallic burner (18) assembly attached to block (1) by means of themounting bracket assembly, metallic burner assembly (18) comprising atleast one oxidant inlet (19), and at least two oxidant outlets (20a) and(20b), first oxidant outlet (20a) opening on said cavity (9) forinjection of the main oxidant, second oxidant outlet (20b) supplyingoxidant to the at least one fuel cavity (7) to initiate the combustionof the fuel close to hot face (4) of refractory burner block (1),

d) a fuel distributor assembly attached to the burner body comprisingone fuel inlet (21) and fuel distribution means (22), fuel distributionmeans (22) extending into the least one fuel cavity (7) for injection ofthe fuel, and providing the at least two fuel streams, and

e) splitting means (23) to distribute the oxidant flow to the at leasttwo oxidant outlets.

FIG. 9b illustrates a sectional view through the fuel injectors 6 (threeare illustrated). For clarity, numerals not necessary for understandingthe figure are not shown. Fuel distribution means 22 is illustrated as aheader, which feeds the three fuel injectors 6.

In other embodiments, such as exemplified in FIG. 10, splitting means(23) is placed outside of metallic burner assembly (18) and in fluidconnection therewith, and oxidant outlets (20a) and (20b) are suppliedwith oxidant from separate inlets (24a) and (24b) originating fromsplitting means (23), splitting means being supplied with oxidantthrough oxidant inlet (25). In this embodiment, a solid plate 26 isnecessary to maintain the separation of primary and secondary oxidantstreams.

In the embodiment of FIG. 11, inlets (24a) and (24b) are fed withindependent oxidant sources, possibly of different chemical compositionand temperature. In this embodiment, a solid plate 26 is necessary tomaintain the separation of primary and secondary oxidant streams.

FIG. 12 illustrates a side sectional view of an alternate embodiment ofthe burner assembly of the present invention, wherein the metallicburner assembly 18 has a shape which is rounded in the vicinity of thefuel injectors (18a). This design may be easier to construct than otherembodiments.

In all embodiments of the invention using fuel injectors, the fuelinjectors may be ceramic or metal, such as stainless steel. Also, theburner assembly components which are metallic may be stainless steelsuch as type 316, or other alloy, such as Hastalloy.

Having described the present invention, it will be readily apparent tothe artisan that many changes and modifications may be made to theabove-described embodiments without departing from the scope of thepresent invention.

What is claimed is:
 1. A burner apparatus for combustion in a combustionchamber of a furnace a fuel with oxygen contained in an oxidant gas,comprising cavity means including fuel injectors for distributing thefuel in at least two adjacent streams injected in the combustion chamberof the furnace and at least one main oxidant cavity ending with anelongated orifice having a major axis along its largest dimension whichis generally parallel to a surface to be heated, the elongated orificesuitable for injecting a major portion of oxidant required for completecombustion of the fuel, wherein the elongated orifice directs the majorportion of oxidant to converge toward the fuel streams in order togenerate a broad flame substantially parallel to a surface to be heated,wherein the inner geometry of the main oxidant cavity comprisespreferably four sections: a first section which is generally cylindricaland having an axis; a second section which is generally cylindrical, ofthe same diameter as the first section; the second section making anangle (B) with the axis of the first section; continuously attached tosecond section is a third section, generally conical with divergingangle (C) ranging from about 10° to about 120°; and a fourth sectionconnecting continuously the third section with the main elongatedorifice.
 2. Apparatus in accordance with claim 1 wherein the fourthsection has a diverging angle equal to the diverging angle (C) of thethird section.
 3. A burner assembly comprising:a) a refractory blockhaving a cold end and a hot end, and further having at least one cavityfor injection of fuel and one cavity for injection of a main oxidant,the cavity for injection of a main oxidant ending on the hot end of therefractory block and having an elongated opening on the hot end, theelongated opening having a major axis generally parallel to material tobe heated, b) a mounting bracket assembly removably attached to the coldend of the refractory block, c) a metallic burner assembly attached tothe refractory block by means of said mounting bracket assembly, themetallic burner assembly comprising at least one oxidant inlet, and atleast two oxidant outlets, a first oxidant outlet opening on said cavityfor injection of main oxidant, a second oxidant outlet supplyingsecondary oxidant to the at least one cavity for injection of fuel toinitiate combustion of the fuel close to the hot face of the refractoryblock, the secondary oxidant further creating a protective layer ofoxidant gas along inside walls of the at least one fuel cavity thatprevents chemical reactions between the refractory block and the fuelthat would eventually damage the refractory block, d) a fuel distributorassembly attached to the metallic burner assembly and comprising onefuel inlet and a fuel distribution means the fuel distribution meanshaving outlet injectors connected thereto and extending into the leastone cavity for injection of the fuel, and providing the at least twofuel streams.
 4. Burner assembly in accordance with claim 3 furthercomprising splitting means to distribute the oxidant flow in the atleast two oxidant outlets.
 5. Apparatus in accordance with claim 3wherein the outlet injectors of the fuel distribution means arestructured so that the fuel streams emanating therefrom make an angleranging from 0° to about 15° between them.
 6. Apparatus in accordancewith claim 3 wherein the outlet injectors of the fuel distribution meansare structured so that fuel emanating therefrom make an angle rangingfrom 0° to about 10° between them.
 7. Apparatus in accordance with claim3 wherein the elongated opening is constructed to direct the mainoxidant stream so that it converges toward the fuel streams with anangle between the main oxidant stream and the fuel streams ranging fromabout 0° to about 45°.
 8. Apparatus in accordance with claim 3 whereinthe elongated opening has an aspect ratio, defined as maximum widthdivided by maximum height, ranging from about 2 to about
 8. 9. Apparatusin accordance with claim 8 wherein the elongated opening aspect ratioranges from about 4 to about
 6. 10. Apparatus in accordance with claim 3wherein the fuel cavities are positioned so that the fuel streamsemanate therefrom substantially parallel to the surface to be heated.11. Apparatus in accordance with claim 3 wherein the fuel cavities areoriented relative to material to be heated with an angle not exceeding+10° or -10°, and the main oxidant cavity is positioned such that themain oxidant stream converges toward these fuel streams and the surfaceto be heated.
 12. Apparatus in accordance with claim 3 includingsecondary oxidant cavities positioned around the at least two fuelstream injectors.